Transitions between the 4f-core-excited states in Ir16+, Ir17+, and Ir18+ ions for clock applications
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Iridium ions near 4f - 5s level crossings are the leading candidates for a new type of atomic clocks with a high projected accuracy and a very high sensitivity to the temporal variation of the fine structure constant a. To identify spectra of these ions in experiment accurate calculations of the spectra and electromagnetic transition probabilities should be performed. Properties of the 4f-core-excited states in Ir16+, Ir17+, and Ir18+ ions are evaluated using relativistic many-body perturbation theory and Hartree-Fock-relativistic method (COWAN code). We evaluate excitation energies, wavelengths, oscillator strengths, and transition rates. Our large-scale calculations included the following set of configurations: 4f(14)5s, 4f(14)5p, 4f(13)5s(2), 4f(13)5p(2), 4f(13)5s5p, 4f(12)5s(2)5p, and 4f(12)5s5p(2) in Pm-like Ir16+; 4f(14), 4f(13)5s, 4f(13)5p, 4f(12)5s(2), 4f(12)5s5p, and 4f (2)5p(2) in Nd-like Ir17+; and 4f(13), 4f(12)5s, 4f(12)5p, 4f(11)5s(2), and 4f(11)5s5p in Pr-like Ir18+. The 5s - 5p transitions are illustrated by the synthetic spectra in the 180-200 angstrom range. Large contributions of magnetic-dipole transitions to lifetimes of low-lying states in the region below 2.5 Ry are demonstrated.